As electromagnetically sensitive implantable medical devices such as defibrillators and pacemakers increase in complexity, the number of interface points to the body, such as attachment points of leads from a cardiac pacemaker or defibrillator to various areas of the heart, also increases. Additionally, as the number of external sources of electromagnetic interference, such as microwave ovens, portable radio transmitters, and the like, increase, their interaction with the implantable medical devices naturally increases as does the need for protection of the sensitive circuitry and external leads of the implantable device against the external sources of electromagnetic interference.
Since the incoming electromagnetic interference energy can arrive unexpectedly as an energy pulse of finite duration and of either positive or negative polarity with respect to the implantable device's circuitry, or as a continuous sinusoidal wave, the protective circuit should be designed to be able to sense and dissipate this incoming energy so that the lead interface input or output circuitry will not be degraded in performance or possibly damaged.
It would be desirable to have the protective device coupled to the circuitry of the implantable device to clamp the incoming energy at a voltage lower than would be destructive to the circuitry of the implantable device (but above the operating voltage of the implantable device, in the case of an output lead of such a device) for shunting the current generated by the external source of energy through an alternate path to the system ground.